A current network theory proposes that local (personal) choices generate global network structure and risk configuration. We base this proposal on the conceptual framework that, in inner city environments, these local choices result in multiple channels of exposure from multiple sources (compound risk); a network structure that facilitates transmission; and geographic proximity that promotes contact with those in the same network. It is the interaction of these three factors that maintains endemicity of HIV and STIs in such settings. Our specific aims are (1) To delineate the behavioral, network, and geographic characteristics of persons who are at risk because of their drug-using and sexual activity in high- and low-burden HIV areas.; (2) To assess the combined influence of these behavioral, network, and geographic factors and their dynamics on the prevalence and incidence of seven sexually transmitted and blood-borne infections (HIV, HCV, HSV-2, Syphilis, Gonorrhea, Chlamydial infection, and Trichomoniasis) in the areas with high and low burden of HIV. Our hypotheses include an expectation of a strong association between social distance (as measured by the geodesic, or shortest distance between two persons in a connected group) and geographic distance (as measured by the metric distance between those two persons); demonstration of the presence and importance of compound risk; confirmation that small networks have structural features that facilitate transmission, and quantitative confirmation of the greater importance of these characteristics in high-burden HIV areas compared to low-burden HIV areas. We will use ethnographic methods to identify 3 "seeds" in each of 5 high-burden and 5 low-burden zip codes, and will use a chain-link design to construct 30 networks of approximately 25 persons each who will be interviewed and tested at baseline and in three follow-up contacts 9 months apart. Their first-degree contacts will be sought, interviewed and tested, to produce a sample of 750 persons at each interview wave. We will use epidemiologic, network, and geospatial analytic methods to examine our underlying hypotheses. If the hypotheses are substantiated, the findings will suggest that categorical approaches to risk reduction in the endemic disease environment are unlikely to be successful; that geographic relationships are factors of basic importance in understanding disease transmission; that geographic proximity supports the need for focused, targeted intervention; and that future modeling efforts relating network structure and dynamics to disease transmission will be enhanced.